Open-faced piston assembly

ABSTRACT

The present disclosure provides an open-faced piston with a circumferential groove into which a piston ring assembly is arranged. Openings at the bottom of the circumferential groove and between a front land of the open-faced piston and the piston face are provided. The openings are arranged to allow for a combustion reaction to propagate through the volume defined between the bottom of the piston ring assembly and the piston face such that at least a portion of an air and fuel mixture located in that volume is reacted.

The present disclosure is directed towards a piston and, moreparticularly, the present disclosure is directed towards a pistonassembly having an open face for reducing unburned hydrocarbonemissions.

BACKGROUND

In a conventional combustion engine, a piston reciprocates inside of acylinder, compressing and expanding a gas mixture. In certain types ofengines, the compressed gas is a mixture of fuel and air. This resultsin a specific problem in which the fuel and air mixture occupies volumesbetween the piston and the cylinder and between a piston surface and aring of the piston, referred to as “crevice volumes.” Because thesurface area to volume ratio of these spaces is high, the fuel and airmixture in a crevice volume is cooled to a low enough temperature suchthat all intended chemical reactions do not take place and the fuel isnot burned during a combustion phase. This unburned fuel is thenexhausted from the engine, resulting in hydrocarbon emissions as well asreduced efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure, in accordance with one or more variousembodiments, is described in detail with reference to the followingfigures. The drawings are provided for purposes of illustration only andmerely depict typical or example embodiments. These drawings areprovided to facilitate an understanding of the concepts disclosed hereinand shall not be considered limiting of the breadth, scope, orapplicability of these concepts. It should be noted that for clarity andease of illustration these drawings are not necessarily made to scale.

FIG. 1 shows a cross-sectional perspective view of a portion of anillustrative engine in accordance with some embodiments of the presentdisclosure.

FIG. 2 is a close up cross-sectional elevation view of a portion of theillustrative engine of FIG. 1.

FIG. 3 is an elevation cross-sectional view of an illustrative enginethat shows the location of crevice volumes in a typical piston and ringconfiguration in accordance with some embodiments of the presentdisclosure.

FIG. 4 is an elevation cross-sectional view of another illustrativeengine that shows the location of crevice volumes in a typical pistonand ring configuration in accordance with some embodiments of thepresent disclosure.

FIG. 5 shows a perspective view of a front portion of an illustrativeopen-faced piston in accordance with some embodiments of the presentdisclosure.

FIG. 6 shows an elevation side view of the open-faced piston of FIG. 5.

FIG. 7 shows a cross-sectional perspective of a portion of an open-facedpiston assembly that includes the open-faced piston of FIGS. 5 and 6 inaccordance with some embodiments of the present disclosure.

FIG. 8 shows an elevation side view of the open-faced piston assembly ofFIG. 7.

FIG. 9 shows a simplified cross-sectional view of a portion of an engineusing an open-faced piston assembly in accordance with some embodimentsof the present disclosure.

DETAILED DESCRIPTION

The present disclosure is applicable towards any type of combustionengine, compressor, or pump having a cylinder with a fuel and anoxidizer mixture, a piston that reciprocates within the cylinder, and apiston ring that separates a high pressure in front of the ring from alower pressure at the back of the ring. For purposes of brevity andclarity, the present disclosure will be described herein in the contextof a combustion engine. It will be understood, however, that thefeatures disclosed herein can be implemented in the context of any othersuitable device, including, for example, a compressor or pump. In someembodiments, the present disclosure can be implemented in acrankshaft-based engine, which typically relies on oil to lubricatepotential piston and cylinder and/or piston ring and cylinder contactpoints, though the present disclosure can be implemented incrankshaft-based engines in which no oil is used. In some embodiments,the present disclosure can be implemented in a free-piston engine, suchas a linear free-piston engine, in which one or more pistons reciprocatewithin one or more cylinders free of any mechanical linkages.Free-piston engines may be operated with or without oil for lubrication.Some examples of free-piston engines are shown in commonly assigned U.S.Pat. No. 8,662,029, issued Mar. 4, 2014 and U.S. Patent ApplicationPublication No. 2016/0208686A1, filed Jan. 15, 2015, both of which arehereby incorporated by reference herein in their entireties. Forpurposes of illustration, and not by way of limitation, the oxidizerused in the above-referenced fuel and oxidizer mixture will be describedas being air. It will be understood that any other suitable oxidizer canbe used in the mixture, including any suitable gas in addition to or inplace of air, any other suitable fluid, or any combination thereof.

FIG. 1 shows a cross-sectional perspective view of a portion of anillustrative engine 100 in accordance with some embodiments of thepresent disclosure. Within cylinder 102 is reciprocating piston 104,including piston ring 106 (also referred to as a sealing ring) arrangedin a circumferential groove of piston 104. FIG. 2 is a close upcross-sectional elevation view of a portion of engine 100 showingvolumes 204 in between a front land 202 of piston 104 and the innersurface of cylinder 102 as well as in between the inner surface ofpiston ring 106 and the outer surface of piston 104. Front land 202 isthe front facing portion of piston 104 and defines the front side 212 ofthe circumferential groove in which piston ring 106 is arranged. Thecircumferential groove is further defined by the illustrated bottom side208 and back side 210. Volumes 204 make up respective crevice volumesinto which fuel and air mixture 206 travels. In a combustion engine,such as in engine 100, when a combustion reaction attempts to propagatethrough a small volume, such as through crevice volumes 204, heattransfer out of the hot gases into the cylinder, piston surfaces, andring cools the fuel and air mixture. If the rate of cooling via heattransfer to the surfaces is faster than the rate of energy release fromthe reaction, then the reaction will quench and stop. As the volume inwhich the reaction is propagating gets physically smaller, the ratio ofsurface area to reaction volume gets larger, resulting in greaterrelative cooling. Below a certain distance between surfaces, it is notpossible for the reaction to propagate through the respective volumes atall. This limiting distance is referred to herein as the quenchdistance. The quench distance is a function of several factors,including, for example, the temperature of the fuel and air mixture, thepressure, type of fuel, the mixture ratio, and the thermal conductivityof the mixture and the material with which it is in contact. Forconditions experienced in an internal combustion engine, the quenchdistance can be, for example, on the order of magnitude of approximately1 mm (10³ microns). By comparison, the distance between the piston outersurface 202 (i.e., at the front land) and inner surface of the cylinder102, and between the front surface 214 of piston ring 106 (i.e., facingthe front side 212 of the piston's circumferential groove) and the frontside 212 of the circumferential groove, is typically on an order ofmagnitude between 10¹ microns and 10² microns. Thus, the combustionreaction cannot typically propagate into, for example, the volumesdefined in front of piston ring 106 (i.e., between the inner surface ofcylinder 102 and the top of front land 202) and below the inner surfaceof the piston ring (i.e., between the inner surface of piston ring 106and the surface of piston 104), and at least some of the fuel locatedthere is not burned. It will be understood that certain aspects of thefigures, including FIG. 2, are not drawn to scale for purposes ofclarity, such as the distance between circumferential groove front side212 and piston ring front surface 214.

FIG. 3 is an elevation cross-sectional view of an illustrative enginethat shows the location of crevice volumes in a typical piston and ringconfiguration in accordance with some embodiments of the presentdisclosure. As illustrated, the crevice volumes, indicated by the shadedvolumes 302 and 304, are divided into two locations: between the topland and the inner surface of the cylinder, and between the piston ringand the circumferential groove in which the piston ring is arranged,respectively. The discussion which follows is with respect to thislatter volume, volume 304 between the piston ring and thecircumferential groove.

In a conventional engine, the crevice volume between the sealing ringand the circumferential groove (e.g., volume 304) can be a source ofsignificant hydrocarbon emissions. Certain design choices applicable toan oil-less piston ring design can further exacerbate this issue. Twoexamples are illustrated in FIG. 4, which shows a particular piston ringdesign in accordance with some embodiments of the present disclosure.First, in designing an oil-free engine, such as an oil-less free-pistonengine, it may be desirable to design an axially thicker piston ring.This would cause the crevice volume between the piston ring and thecircumferential groove to be increased. For example, comparing thepiston rings shown in FIG. 3 and FIG. 4, it is apparent that the crevicevolume 404 is larger in FIG. 4 as compared to crevice volume 304 in FIG.3 due to the increased axial width of the piston ring used in the designshown in FIG. 4 relative to that of FIG. 3. Second, certain materialsused for a piston ring (e.g., carbon graphite, polymers such aspolyether ether ketone (PEEK), polytetrafluoroethylene (PTFE) (e.g.,Teflon), and Molybdenum disulfide (MoS2)) might wear at a relativelyhigh rate. This might be particularly relevant when, for example,designing an oil-less engine, such as an oil-less free-piston engine,which would typically use a material for the piston ring that wearsrelatively quickly (when compared to, for example, typical hard-facedoil-lubricated piston rings). As material is removed from the pistonring due to this wear, additional crevice volume 402 between the ringand groove opens up where the ring material used to be. This isillustrated by the dashed lines in FIG. 4, which indicate the worn awayportion of the piston ring being replaced with additional crevicevolume.

In some embodiments of the present disclosure, the piston face ismodified in order to open the volume between the piston ring and thecircumferential groove to the cylinder volume. Openings between thepiston face and the circumferential groove in which the piston ring isarranged are introduced that allow the combustion reaction to reach thevolume between the inner surface of the ring and the circumferentialgroove. These communicating openings have a characteristic length largerthan the quench distance in order to allow the reaction to propagateinto the target volume (e.g., where the crevice volumes would be absentthe openings) and burn the fuel located there. For example, when thequench distance is 1 mm, the characteristic length of these openingswill be greater than 1 mm.

In some embodiments, the front land is necessary in order to contain thepiston ring on the piston under the effect of friction and accelerationloads, which could otherwise throw the piston ring away from the ringseating area on the piston. Therefore, a design tradeoff arises in thatit is desirable to leave the passage between the front land and thepiston face as open as possible to allow complete burning of fuellocated between the piston ring and the piston, but also necessary tomechanically attach the front land to the piston with sufficientstrength. In some embodiments, this can be achieved using a design asillustrated in FIGS. 5 and 6.

FIG. 5 shows a perspective view of a front portion of an illustrativeopen-faced piston 500 in accordance with some embodiments of the presentdisclosure. FIG. 6 shows an elevation side view of the open-faced pistonof FIG. 5. Whereas in a conventional piston design the surface of apiston face extends fully radially outwards to the edge of the frontland, in the design illustrated in FIGS. 5 and 6, the piston face 508extends radially outward only partially, leaving an opening 504 betweenitself and front land 502. In addition, in the illustrated design,spaced openings 506 are provided along circumferential groove 512 inwhich the piston ring would be arranged.

Structural webs 510 are provided in order to attach the front land tothe back side of circumferential groove 512. Structural webs 510 alsoserve to partially define the bottom side of circumferential groove 512.More particularly, the bottom side of circumferential groove 512 isdefined by structural webs 510 as well as by spaced openings 506.Openings 504 and 506 are sized and arranged such that a minimum openingalong a gas path between each opening and an inner surface of the pistonring assembly is at least a quench distance of the air and fuel mixture.For example, the openings can be sized between 1 mm and 10 mm or evengreater (i.e., where the quench distance is less than 1 mm) In otherexamples, the openings can be sized less than 1 mm so long as thedimensions of the openings are still greater than the quench distance ofthe air and fuel mixture. It will be understood that the illustrateddesign is merely exemplary. Any other suitable design that providesopenings for allowing a combustion reaction to propagate into thecrevice volume defined between the inner surface of a piston ring andthe bottom of the circumferential groove may be used in accordance withthe principles of the present disclosure. For example, while FIGS. 5 and6 show openings 506 as being substantially equivalent to one another interms of geometry and spacing, they need not be. Openings 506 may of anysuitable size and shape. They may be all substantially equivalent or,alternatively, one or more of the openings 506 may be different in size,shape, or both relative to one or more of the remaining openings 506.The size and shape of the structural webs 510 between openings 506(i.e., the spacing and design) can be substantially equivalent to oneanother, or, alternatively, one or more of structural webs 510 may bedifferent in size, shape, or both relative to one or more of theremaining structural webs 510. In some embodiments, openings 506 maycover a majority of the bottom of circumferential groove 512.

Opening 504, while illustrated as a single symmetrical opening, may, insome embodiments, be implemented as two or more openings having anysuitable geometrical properties. For example, in some embodiments, thefront face of front land 502 may extend radially inward and connect topiston face 508 to create additional structural webs substantiallyorthogonal in orientation to illustrated structural webs 510. It will beunderstood that these additional structural webs need not besubstantially orthogonal to illustrated structural webs 510 (i.e.,depending on the location of piston face 508 relative to front 502).With this additional structural support, these embodiments might allowfor openings 506 to be greater in number, size, or both. In one suitableapproach, a single opening may be used in place of multiple openings 506such that the bottom of the circumferential groove is entirely open. Inthis approach, at least one structural web either between front land 502and piston face 508, axially across the bottom of circumferential groove512, or both would be needed in order to attach front land 502 to therest of piston 500.

FIGS. 7 and 8 show cross-sectional perspective and side elevation views,respectively, of a portion of an open-faced piston assembly thatincludes the open-faced piston of FIGS. 5 and 6 and piston ring assembly702 in accordance with some embodiments of the present disclosure.Piston ring assembly 702, having piston ring inner surface 704 isarranged in the circumferential groove and is physically contained inthe groove by virtue of front land 502 defining the front side of thegroove. At the same time, piston ring inner surface 704 is exposed tothe air and fuel mixture in the cylinder over at least a majority of itscircumference and axial width. During combustion, the reactionpropagates through to at least a portion of piston ring inner surface704 causing the fuel and air mixture located in the volume definedbetween piston ring inner surface and piston face 508 to be at leastpartially reacted. This at least partial reaction causes at least morethan a negligible amount of the fuel and air mixture in this volume tobe reacted. In some embodiments, the amount of the fuel and air mixturein this volume that is reacted is at least a majority of the fuel andair mixture. In some embodiments, the amount of the fuel and air mixturein this volume that is reacted is at least substantially all of the fueland air mixture.

Piston ring assembly 702 may include a single continuous piston ringmade of any suitable material and dimensions. In some embodiments,piston ring assembly 702 may include two or more portions of a pistonring that are coupled, able to move independently of each other (e.g.,in the radial or axial direction), or both. As illustrated, piston ringassembly 702 is a single contiguous ring. The smallest extent of theopenings between piston face 508 and front land 502 is greater than 1mm, ensuring that the fuel and air mixture located between piston ringinner surface 704 and piston 500 is reacted when the quench distance is1 mm or less. The structural web geometry ensures the front land remainsattached to the piston despite cyclic acceleration loads from pistonring 702 pushing forward on front land 502. In some embodiments, pistonring assembly 702 may be made at least partially of a solid lubricatingmaterial and used, for example, in the manner illustrated in FIG. 4without concern of an increasing crevice volume because of the benefitof openings 504 and 506.

In some embodiments, circumferential groove 512 extends around theentirety of the circumference of piston 500. In some embodiments,circumferential groove 512 extends only partially around thecircumference of piston 500. Circumferential groove 512 may be a singlecontinuous groove or may be two or more discrete grooves (i.e., eachhaving its own respective piston ring portion) arranged serially aroundthe circumference of piston 500. When circumferential groove 512includes more than one groove, the multiple grooves, in someembodiments, go entirely around the circumference of piston 500 and maybe separated by any suitable distance from each respective facing end.It will be understood that while the present disclosure describesembodiments in which there is a single circumferential groove 512, insome embodiments there may be multiple circumferential grooves that arearranged in parallel axially along the length of piston 500, each ofwhich accommodates a respective piston ring assembly. For example, asillustrated in FIGS. 5-8, an additional circumferential groove 514 isprovided that is configured to accommodate an addition piston ring (notshown). Circumferential groove 514 is arranged in parallel tocircumferential groove 512 and is located behind circumferential groove512.

FIG. 9 shows a simplified cross-sectional view of a portion of an engine900 using an open-faced piston assembly in accordance with someembodiments of the present disclosure. Piston assembly 912 reciprocatesin cylinder 902. Combustion reaction 914 originating from a combustionsection of cylinder 902 is able to propagate and react through opening910 into the volume defined between the inner surface of ring 906 andpiston 904. It will be understood, therefore, that the features of thepresent disclosure provide for a combustion reaction to propagate intothis volume. It will further be understood that, while a combustionreaction propagates into the volume defined between the inner surface ofring 906 and piston 904, that not necessarily all of the fuel and airmixture in that volume is reacted, but at least a portion, greater thana negligible portion of the mixture (for example, in some embodiments,at least a majority of the mixture, or at least substantially all of themixture). Front land 908 is attached to the piston 904 by structuralwebs (not shown in FIG. 9) and is able to hold ring 906 in place. Itwill be understood that engine 900 may be implemented using any suitablearrangement. For example, more than one cylinder may be used, more thanone piston in one or more cylinders may be used, or any other suitablearrangement may be implemented using the features of the presentdisclosure.

The foregoing is merely illustrative of the principles of thisdisclosure, and various modifications may be made by those skilled inthe art without departing from the scope of this disclosure. Theabove-described embodiments are presented for purposes of illustrationand not of limitation. The present disclosure also can take many formsother than those explicitly described herein. Accordingly, it isemphasized that this disclosure is not limited to the explicitlydisclosed methods, systems, and apparatuses, but is intended to includevariations to and modifications thereof, which are within the spirit ofthe following claims.

1-27. (canceled)
 28. A piston assembly comprising: a piston, comprising:a circumferential groove that extends at least partially around an outersurface of the piston, and a plurality of openings located along thecircumferential groove; and a piston ring assembly arranged within thecircumferential groove, wherein the plurality of openings is configuredto allow a combustion reaction to propagate into a volume definedbetween an inner surface of the piston ring assembly and the piston. 29.The piston assembly of claim 28, wherein each of the plurality ofopenings is sized and arranged such that a minimum opening along a gaspath between each opening and the inner surface of the piston ringassembly is at least a quench distance of an oxidizer and fuel mixturein the volume.
 30. The piston assembly of claim 29, wherein the quenchdistance is approximately 1 mm.
 31. The piston assembly of claim 28,wherein the distance between the outer surface of the piston and theinner surface of the piston ring assembly is greater than 1 mm.
 32. Thepiston assembly of claim 28, wherein the piston further comprises: afront land; a piston face; and at least one structural web that attachesthe front land to the piston face.
 33. The piston assembly of claim 28,wherein the plurality of openings comprises a plurality of spacedopenings along the circumferential groove that are substantiallyequidistant from each other.
 34. The piston assembly of claim 28,wherein the plurality of openings comprises a plurality of spacedopenings along the circumferential groove that are of substantiallyequivalent geometry to one another.
 35. The piston assembly of claim 28,wherein the plurality of openings is configured to allow at least amajority of an oxidizer and fuel mixture in the volume to react.
 36. Thepiston assembly of claim 28, wherein the plurality of openings comprisesa plurality of spaced openings along the circumferential groove thatcomprise a majority of a surface area of a bottom of the circumferentialgroove.
 37. The piston assembly of claim 28, wherein the piston assemblyis a free-piston assembly.
 38. The piston assembly of claim 28, whereinthe piston assembly is configured for oil-less operation.
 39. The pistonassembly of claim 28, wherein the piston ring assembly comprises a solidlubricating material.
 40. The piston assembly of claim 28, wherein thecircumferential groove is a first circumferential groove, the pistonassembly further comprising a second circumferential groove arranged inparallel to and behind the first circumferential groove.
 41. The pistonassembly of claim 28, wherein the combustion reaction propagates from acombustion section of an engine in which the piston assembly isconfigured to translate.
 42. An engine comprising: at least one cylindercomprising: a combustion section, and an inner cylinder surface; and atleast one piston assembly arranged within the cylinder, the at least onepiston assembly comprising: a piston comprising: a circumferentialgroove that extends at least partially around an outer surface of thepiston, and a plurality of openings located along the circumferentialgroove, and a piston ring assembly arranged within the circumferentialgroove, wherein the plurality of openings is configured to allow acombustion reaction to propagate from the combustion section into avolume defined between an inner surface of the piston ring assembly andthe piston.
 43. The engine of claim 42, wherein an oxidizer and fuelmixture is able to pass from the combustion section into the volumedefined between the inner surface of the piston ring assembly and thepiston.
 44. The engine of claim 42, wherein each of the plurality ofopenings is sized and arranged such that a minimum opening along a gaspath between each opening and an inner surface of the piston ringassembly is at least a quench distance of an oxidizer and fuel mixture.45. The engine of claim 44, wherein the quench distance is approximately1 mm.
 46. The engine of claim 42, wherein the distance between the outersurface of the piston and the inner surface of the piston ring assemblyis greater than 1 mm.
 47. The engine of claim 42, wherein the pistonfurther comprises: a front land; a piston face; and at least onestructural web that attaches the front land to the piston face.
 48. Theengine of claim 42, wherein the plurality of openings comprises aplurality of spaced openings along the circumferential groove that aresubstantially equidistant from each other.
 49. The engine of claim 42,wherein the plurality of openings comprises a plurality of spacedopenings along the circumferential groove that are of substantiallyequivalent geometry.
 50. The engine of claim 42, wherein the pluralityof openings is configured to allow at least a majority of an oxidizerand fuel mixture in the volume to react.
 51. The engine of claim 42,wherein the piston assembly is a free-piston assembly.
 52. The engine ofclaim 42, wherein the piston assembly is configured for oil-lessoperation.
 53. The engine of claim 42, wherein the plurality of openingscomprises a plurality of spaced openings that comprise a majority of asurface area of a bottom of the circumferential groove.
 54. The engineof claim 42, wherein the piston ring assembly comprises a solidlubricating material.
 55. The engine of claim 42, wherein thecircumferential groove is a first circumferential groove, the pistonfurther comprising a second circumferential groove arranged in parallelto and behind the first circumferential groove.